Equation-of-state-insensitive measure of neutron star stiffness

TL;DR

This paper identifies a nearly universal relation linking neutron star properties like compactness, moment of inertia, and tidal deformability to a measure of nuclear matter stiffness, aiding astrophysical and nuclear physics insights.

Contribution

It uncovers a new equation-of-state-insensitive relation connecting neutron star macroscopic properties to a measure of nuclear matter stiffness, enhancing understanding of neutron star interiors.

Findings

Discovered a tight correlation between neutron star properties and central pressure-to-energy density ratio.

Quantified the universality of the relations across different equations of state.

Explored implications for interpreting neutron star observations.

Abstract

Universal relations (i.e., insensitive to the equation of state) between macroscopic properties of neutron stars have proven useful for a variety of applications -- from providing a direct means to extract observables from data to breaking degeneracies that hinder tests of general relativity. Similarly, equation-of-state-insensitive relations directly connecting macroscopic and microscopic properties of neutron stars can potentially provide a clean window into the behavior of nuclear matter. In this work, we uncover a tight correlation between certain macroscopic properties of a neutron star - its compactness $C$, moment of inertia $\bar{I}$ and tidal deformability $\bar{\Lambda}$ - and the ratio $\alpha_c \equiv p_c/\epsilon_c$ of central pressure to central energy density, which can be interpreted as a mean notion of the stiffness of nuclear matter inside that object. We describe interesting properties of this stiffness measure, quantify the (approximate) universality of the $\alpha_c - C/\bar{I}/\bar{\Lambda}$ relations, and explore its consequences in the face of recent and future neutron star observations.